1. Field of the Invention
The present invention relates to a semiconductor laser device used for optical disks and the like and to a method of manufacturing the same. More specifically, the present invention relates to a semiconductor laser device with a window structure having superior high-output operation characteristic, and to a method of manufacturing the same.
2. Description of the Background Art
Recently, various types of semiconductor lasers have come to be widely used as light sources for optical disks. Particularly, high-output semiconductor lasers are used as light sources for writing to disks for DVD (Digital Versatile Disc) players or DVD-RAM (Random Access Memory) drives, and still higher output is desired.
One factor that limits higher output of a semiconductor laser is COD (Catastrophic Optical Damage) generated in an active region near an end surface of a laser resonator, as optical output density increases.
A COD is generated as the active region near the end surface of the laser resonator comes to be an absorption region absorbing the laser beam. There are many non-radiative recombination centers on the end surface of the laser resonator as referred to as surface state or interface state, and carriers introduced to the active layer near the end surface of the laser resonator are lost because of the non-radiative recombination centers. Therefore, injection carrier concentration in the active layer near the end surface of the laser resonator is smaller than at a central portion. As a result, the active region near the end surface of the laser resonator comes to be an absorption region absorbing the laser beam of the wavelength resulting from the high carrier concentration at the central portion. Increase in optical output density results in local heat build up at the absorption region, causing higher temperature and smaller band gap energy. This yields a positive feedback that the absorption coefficient further increases and the temperature increases, and the temperature of the absorption region near the end surface of the laser resonator eventually reaches the melting point of the semiconductor material, generating the COD.
As one method of attaining higher output of a semiconductor laser, a method has been proposed in which an active layer of a multi-quantum well near the end surface of the laser resonator is disordered to provide a window structure, in order to improve characteristic level against the COD (Japanese Patent Laying-Open Nos. 7-162086, 11-284280).
FIGS. 18A to 18C are schematic illustrations representing the structure of the semiconductor laser device having the window structure. Specifically, FIG. 18C is a perspective view including the emission end surface, in which the reference character B represents window regions near the end surface of the laser resonator, and reference character A represents laser oscillation region (inner region) other than the window regions. FIG. 18A is a cross section of region A, and FIG. 18B is a cross section of region B.
As shown in FIGS. 18A to 18C, a conventional semiconductor laser device having a window structure includes, on an n type GaAs substrate 1001, an n type AlGaInP clad layer 1002, an MQW active layer (an active layer of multi-quantum well having a multi-quantum well structure with alternately stacked barrier and well layers sandwiched by optical guide layers) 1003, a p type AlGaInP clad layer 1004 and a p type GaAs first contact layer 1005 stacked in order. The p type AlGaInP clad layer 1004 and the p type GaAs first contact layer 1005 form a stripe shaped ridge 1008 to be a laser resonance wave guide path, and an n type GaAs current block layer 1009 is arranged to fill the side surfaces of the ridge. On ridge 1008 or n type GaAs current block layer 1009, a p type GaAs second contact layer 1010 and a p side electrode 1011 are arranged, and below the n type GaAs substrate 1001, an n side electrode 1012 is arranged.
At ridge 1008 in the region near the end surface of the laser resonator, a window structure region 1013 is formed, which is disordered by diffusion of Zn atoms from a ZnO film as a source of impurity diffusion.
FIGS. 19 to 22 represent steps of manufacturing a conventional semiconductor laser device having the window structure. First, as shown in FIG. 19, on n type GaAs substrate 1001, n type AlGaInP clad layer 1002, un-doped MQW (Multi-Quantum Well) active layer 1003, p type AlGaInP clad layer 1004 and p type GaAs first contact layer 1005 are formed successively. Here, p type AlGaInP clad layer 1004 is doped with Zn atoms as a p type impurity.
Next, as shown in FIG. 20, on the entire surface of the wafer, ZnO film 1006 as a source of impurity diffusion is formed. Thereafter, a stripe shaped resist mask 1021 is formed to extend in a direction horizontal to the end surface of the resonator of the semiconductor laser, and ZnO film 1006 that has been formed on the laser oscillation region other than the window region B is etched away.
Thereafter, resist mask 1021 is removed, and SiO2 film 1007 is formed on the entire surface of the wafer. Then, a stripe shaped resist mask (not shown) is formed to extend in a direction vertical to the end surface of the resonator of the semiconductor laser, and SiO2 film 1007 formed on the current block region etched. Then, the resist mask (not shown) is removed, and portions of p type GaAs first contact layer 1005 and p type AlGaInP clad layer 1004 are etched to form ridge 1008.
Thereafter, as shown in FIG. 22, n type GaAs current block layer 1009 is grown for filling, SiO2 film 1007 is removed, GaAs second contact layer 1010 is grown, and heat treatment is performed for about 30 minutes at a temperature of 500° C. or higher, which is the temperature at the time of growing the current block layer, so that Zn atoms are thermally diffused from ZnO film 1006 as a source of impurity diffusion into p type GaAs first contact layer 1005, p type AlGaInP clad layer 1004 and MQW active layer 1003. Consequently, MQW active layer 1003 is disordered, resulting in a window structure region 1013. Thereafter, p type electrode 1011 and n type electrode 1012 are formed, the wafer is cleaved, and the semiconductor device shown in FIG. 18A to 18C is obtained.
As described above, in the conventional semiconductor laser device having a window structure, in the window region B formed near the end surface of the laser resonator, ZnO film 1006 is formed such that the band gap energy becomes larger than that corresponding to the laser oscillation wavelength, and Zn atoms are thermally diffused into MQW active layer 1003.
Further, in order to promote diffusion of Zn atoms to the window region B and to suppress diffusion of Zn atoms to the active region in inner region A, a proposal has been made to provide a layer containing Si atoms, on the film 1006 as a source of impurity diffusion (Japanese Patent Laying-Open No. 2001-94206).
Here, in order to have the band gap energy of the active region near the light emission end surface larger than the band gap energy corresponding to laser oscillation wavelength, annealing at a high temperature over a long period of time is necessary. According to the conventional method described above, the impurity atoms diffused from ZnO film 1006 as a source of impurity diffusion and the impurity atoms doped in p type AlGaInP clad layer 1004 are both Zn atoms that have large diffusion coefficient in AlGaInP based material. Accordingly, Zn atoms diffuse into MQW active layer 1003 even in the inner region, resulting in increase of driving current or driving voltage at a high-output operation and in lower reliability over a long period of time. Even when the method proposed in Japanese Patent Laying-Open No. 2001-94206 is used, diffusion of Zn atoms into the active region in the inner region A cannot sufficiently be suppressed from the reason described above.
Diffusion of Zn atoms into the MQW active region in the inner region can be suppressed by lowering the annealing temperature or shortening annealing time. This, however, results in insufficient diffusion of Zn atoms into the MQW active layer 1003 in the window region B. This leads to absorption of much more laser beam at the active layer near the end surface of the laser resonator, making generation of COD more likely. Further, maximum optical output at the time of high-output driving would be lower and satisfactory long-life reliability would not be attained.